This invention relates to the construction of furnaces, and, more particularly, to a furnace for sintering powder materials held together with an organic binder.
Powder metallurgical processing is a technique for manufacturing metal (or ceramic) articles. Powders of metals or ceramics are molded by metal injection molding or pressed into the desired preform shape of the finished article. This preform is heated to a temperature at which the powders bind together, or sinter, either by solid state or liquid phase diffusion. Preparation of parts by sintering has important advantages over casting or machining techniques, which include a highly uniform microstructure, low cost production of large numbers of parts, and little waste when the sintered piece is final machined to a useful article. When the forming and sintering operations are conducted properly, articles produced from powders can have properties superior to those of cast or wrought articles.
The powders are formed into the proper shape of the finished article, but must be held in this "green" or unsintered form until sintering can be completed. An organic-based binder is therefore mixed with the powders prior to pressing or molding, and stays with the powders when they are pressed or molded. The binder acts much like a glue to hold the powders in place until they are heated for the sintering operation. The organic binder must be removed from the powder compacts immediately prior to, or during sintering. If the organic binder remains mixed with the powder, it prevents full densification during sintering and results in reduced mechanical properties of the sintered part.
Most sintering cycles for metal powders having organic binders include a preheat period at relatively low temperature. During the preheat period, the organic binders are vaporized and driven from the powder article. The preheat temperature is selected such that a small amount of solid state sintering occurs as the organic material is driven out, so that the compact holds its shape until sintering can be completed at higher temperature, but not so much sintering occurs that the organic vapor cannot escape through open surface porosity.
This type of sintering procedure is widely practiced, but there is a continuing problem of removing the organic material without fouling the interior of the furnace. Some sintering operations use two furnaces, one operating at low temperature to remove the organic material and a second sintering furnace operating at high temperature to effect sintering of the article. Other furnaces use a high gas flow of a sweep gas to flush the organic vapor from the furnace during its evolution. Other furnaces are designed to be easily cleaned, and conduct the sintering without concern for evolution of the organic vapors. However, all of the existing sintering furnaces suffer from an inability to handle high organic loadings, while remaining clean, and an inability to prevent redeposition of the organic material upon the sintered article during and after the sintering process.
There is a need for an improved furnace that permits sintering at high temperatures of 2000.degree. F. and greater, but also can handle high organic loadings during the vaporization of the binder in the preheating step. The present invention fulfills this need, and further provides related advantages.